The present embodiments relate to activating a magnetic resonance system having a transmit antenna arrangement including a plurality of independent high-frequency transmit channels with a respectively assigned transmit antenna.
In a magnetic resonance tomography system (e.g., a magnetic resonance system), a body to be examined may be exposed, with the aid of a basic field magnet system, to a relatively powerful basic magnetic field (e.g., a B0 field of 0.2 or 7 Tesla). A magnetic field gradient is also applied with the aid of a gradient system. A high-frequency transmit system is used to emit high-frequency excitation signals (HF signals) using suitable antenna devices, with the intention of tilting the nuclear spins of specific atoms excited in a resonant manner by the high-frequency field with spatial resolution through a defined flip angle in relation to the magnetic field lines of the basic magnetic field.
The high-frequency magnetic field emitted in the form of individual pulses or pulse trains is also referred to as the B1 field. Such magnetic resonance excitation (MR excitation) by magnetic high-frequency pulses or the resulting flip angle distribution is also referred to in the following as nuclear magnetization or magnetization. As the nuclear spins relax, high-frequency signals (e.g., magnetic resonance signals) are emitted. The emitted high-frequency signals are received by suitable receive antennas and further processed. The desired image data may be reconstructed from the raw data acquired in this manner.
The emission of the high-frequency signals for nuclear spin magnetization may be effected using a whole body coil or body coil. A whole body coil may have the structure of a birdcage antenna including a number of transmit rods that are disposed, running parallel to the longitudinal axis, around a patient chamber of the tomography unit, in which a patient is accommodated during the examination. The antenna rods are connected capacitively to one another in a ring shape at ends of the antenna rods. Local coils in proximity to the body are increasingly used to emit MR excitation signals. The magnetic resonance signals may be received by the local coils. Alternatively or additionally, the magnetic resonance signals may be received by the body coil.
With more recent magnetic resonance systems, individual HF signals may be applied to individual transmit channels. A multichannel pulse that includes, as described above, a number of individual high-frequency pulses that may be emitted in a parallel manner via the different independent high-frequency transmit channels, is emitted. Such a multichannel pulse train (e.g., a pTX pulse because of the parallel emission of the individual pulses) may be used, for example, as an excitation, refocusing and/or inversion pulse. An antenna system having a number of independently activatable antenna components or transmit channels may also be referred to as a transmit array, regardless of whether the antenna system is a whole body antenna or an antenna arrangement close to the body.
Such pTX pulses or pulse trains made up of the pTX pulses may be determined beforehand for a specific planned measurement (e.g., the pulse shape and phase are determined). The pulses are to be emitted on the individual transmit channels with the specific planned measurement.
To plan the HF pulses, the user predefines a target magnetization (e.g., a desired flip angle distribution with spatial resolution) that is used as a setpoint value within the target function. The appropriate HF pulses for the individual channels are calculated so that the target magnetization is reached as far as possible. The basis for this is the Bloch equation
                                          ⅆ            M                                ⅆ            t                          =                              γ            ·            M                    ×          B                                    (        1        )            which describes the magnetization established by a magnetization vector M in a magnetic field B. γ is the gyromagnetic ratio of the nucleus to be excited (e.g., for normally excited hydrogen, γ=42.58 MHz/T).
The pulse shape may be calculated such that a pulse of a specified length is discretized into a number of very short time steps. Time steps of 1 to 1000 μs duration may be used (e.g., a pulse of 10 to 20 ms includes more than 1000 time steps).
For small flip angles, the Bloch equation produces a linear equation systemA·b=mdes  (2)where mdes stands for the vector of the spatially discretized target magnetization, the vector b stands for the time discretization of the HF pulses, and A is a matrix including the linear relations resulting from the discretization of the linearized solution of the Bloch equations between the vector mdes and the vector b. The solution of this equation system produces, for each of the time steps, a complex pulse value with a real and an imaginary part that represent the voltage amplitude and phase of the pulse for the activation of the magnetic resonance system.
The relationship between an HF pulse radiated in a resonant manner with field strength B1 and the flip angle α thereby achieved is defined by the equation
                    α        =                              ∫                          t              =              0                        t                    ⁢                      γ            ·                                          B                1                            ⁡                              (                t                )                                      ·                                                  ⁢                          ⅆ              t                                                          (        3        )            where γ is the gyromagnetic ratio, and τ is the action period of the high-frequency pulse. The flip angle achieved by an emitted HF pulse and therefore the strength of the magnetic resonance signal are a function not only of the duration of the HF pulse, but also of the strength of the radiated B1 field.
The multichannel transmit systems or transmit arrays set out above are also used, for example, to stamp (e.g., mark) a specified spatial distribution in amplitude and phase on the transverse magnetization after excitation. It is disadvantageous that with such selective transmission, the HF pulse becomes very long. Selective transmission refers, for example, to selective excitation. For example, the anatomical shape to be examined may be stamped. The liver or prostate, for example, may thus be excited or magnetized selectively, which may shorten the examination time significantly. In order to make the long HF pulse sufficiently short, the k-space may be traversed very quickly during transmission. Fast traversing of the k-space may be provided when the highest possible gradient amplitudes are used. High gradient amplitudes have the disadvantage that the high gradient amplitudes increase the required transmit bandwidth of the high-frequency signal. The transmit bandwidth fBW is calculated from the gradient G active during transmission and the extension D of the object, on which the desired magnetization is to be stamped, as:
                              f          BW                =                              γ                          2              ⁢              π                                ·          G          ·          D                                    (        4        )            
If a magnetization is to be stamped, for example, with a gradient of 40 mT/m over a spatial region of the extension 500 mm, a bandwidth of approximately 850 kHz is used for 1H imaging.
Signal transmission components such as amplifiers and filters that allow a high transmission bandwidth and have high amplitude linearity permit phase-true transmission and are symmetrical with respect to amplitude and phase, are complex to manufacture and are correspondingly expensive. For example, the real-time regulators present in the transmit chain in modern magnetic resonance systems, which keep the HF amplitude constant, restrict the bandwidth. The real-time regulators may have a very restricted bandwidth on the order of 100 kHz, for example, due to the signal processing time. The real time regulators may be embodied as digital.
While this typical 100 kHz bandwidth of the regulators is sufficient to produce amplitude-modulated HF pulses with bandwidths in the region of several kHz without distortion during standard magnetic resonance imaging, the cited regulator bandwidth is much too low for the example cited. Either this prevents the use of a regulator, or the maximum gradient amplitude when the k-space is traversed during transmission is to be restricted. The HF pulse is thus lengthened over the entire period.